This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-390518, filed Dec. 21, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) which uses a magneto resistive effect to store data xe2x80x9c1xe2x80x9d, xe2x80x9c0xe2x80x9d.
2. Description of the Related Art
In recent years, a large number of memories have been proposed in which data is stored according to a new principle. Examples of the memories include a magnetic random access memory which uses a tunneling magneto resistive (hereinafter referred to as TMR) effect to store data xe2x80x9c1xe2x80x9d, xe2x80x9c0xe2x80x9d.
A known proposal of the magnetic random access memory is, for example, xe2x80x9cA 10 ns Read and Write Nonvolatile Memory Array using a Magnetic Tunnel Junction and FET Switch in each Cellxe2x80x9d, ISSCC2000 Technical Digest p.128 by Roy Scheuerlein et al.
The magnetic random access memory stores the data xe2x80x9c1xe2x80x9d, xe2x80x9c0xe2x80x9d by a TMR element. In a basic structure of the TMR element, a tunneling barrier is held between two magnetic layers (ferromagnetic layers).
The data stored in the TMR element is determined by judging whether magnetizing states of two magnetic layers are parallel or antiparallel.
Here, term xe2x80x9cparallelxe2x80x9d means that two magnetic layers have the same magnetizing direction, and xe2x80x9cantiparallelxe2x80x9d means that two magnetic layers are opposite to each other in the magnetizing direction.
Usually, an antiferromagnetic layer is disposed on one (fixed layer) of two magnetic layers. The antiferromagnetic layer is a member for fixing the magnetizing direction of the fixed layer. Therefore, in actual, the data (xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d) to be stored in the TMR element is determined in accordance with the magnetizing direction of the other one (free layer) of two magnetic layers.
When the magnetizing state of the TMR element is parallel, the tunneling barrier held between two magnetic layers constituting the TMR element has a lowest resistance. For example, this state is regarded as state xe2x80x9c1xe2x80x9d. Moreover, when the magnetizing state of the TMR element is antiparallel, the tunneling barrier held between two magnetic layers constituting the TMR element has a highest resistance. For example, this state is regarded as state xe2x80x9c0xe2x80x9d.
For a cell array structure of the magnetic random access memory, at present various structures have been studied from viewpoints of enlargement of a memory capacity, stabilization of a write/read operation, and the like.
For example, at present, a cell array structure is known in which one memory cell is constituted of one MOS transistor and one TMR element (or a magnetic tunnel junction (MTJ) element). Moreover, another magnetic random access memory is also known in which the cell array structure is disposed and two memory cell arrays are used to store one bit data in order to stabilize the read operation.
However, in these magnetic random access memories, it is difficult to enlarge the memory capacity. This is because one MOS transistor corresponds to one TMR element in these cell array structures.
According to an aspect of the present invention, there is provided a magnetic random access memory comprising:
a plurality of memory cells which use a magneto resistive effect to store data; a read select switch connected in common to one end of each of the plurality of memory cells; and a plurality of bit lines which are disposed for the plurality of memory cells, and which extend in a first direction, wherein the other ends of the plurality of memory cells are independently connected to one of the plurality of bit lines.
According to another aspect of the present invention, there is provided a manufacturing method of a magnetic random access memory comprising:
forming a read select switch in a surface region of a semiconductor substrate; forming a plurality of bit lines which extend in a first direction on the read select switch; forming a plurality of MTJ elements connected to the plurality of bit lines right above the plurality of bit lines; forming an electrode connected in common to the plurality of MTJ elements, and connected to the read select switch right above the plurality of MTJ elements; and forming a write line which extends in a second direction intersecting the first direction in the vicinity of the plurality of MTJ elements.